1. Field of the Invention
The present invention relates to method and device used in apparatuses such as light beam scanning apparatuses for reading or recording images with a laser beam to be scanned on a recording or recorded surface while focusing the laser beam thereon.
2. Related Art
In such light beam scanning apparatuses, the diameter of the laser beam focused on the recording or recorded surface (hereinafter, simply referred to as focused beam diameter) needs to be changed according to the image recording or reading density. In other words, the focused beam diameter must be changed according to the scanning density. For example, the focused beam diameter needs to be expanded when the image recording (reading) density is reduced (i.e., when the scanning density is reduced).
Conventionally, two types of methods for changing a focused beam diameter have been proposed. The first method uses an aperture stop (see Unexamined Japanese Patent Publication (KOKAI) Nos. 141662/1981 and 144850/1983). The second method is to control the focused beam diameter by changing the magnification of an optical system. In the first method, the aperture stop is inserted in a collimated (parallel) portion of the light beam to limit the diameter of light flux of the collimated beam (parallel beam). In other words, the first method uses eclipse or shading to expand the focused beam diameter.
A focused beam diameter d is generally given by the following equation: EQU d=K.lambda.f/D,
where K is a coefficient, .lambda. is a beam wavelength, f is a focal length of a scanning lens (focusing lens) and D is a diameter of light flux of a beam to be incident on the scanning lens (hereinbelow, referred to as an incident beam diameter). The beam diameter d also becomes equal to a diameter of a circle formed by connecting points where a beam power of 1/e.sup.2 (approximately equal to 0.135) to the maximum beam power point (the center point of the beam) is obtained.
As apparent from the above equation, the incident beam diameter D is made small to expand the focused beam diameter d. Stated more strictly, the first method is to control the beam diameter d by changing the incident beam diameter D. Assuming that an incident beam diameter when d=10 .mu.m is D.sub.0, when the focused beam diameter d needs to be expanded to 13 .mu.m, an aperture stop capable of reducing the incident beam diameter D to D=10/13=0.77D.sub.0 must be inserted in a portion of the incident beam (parallel beam).
In the second method, the collimated beam diameter to be incident on the scanning lens is generally large, and therefore a beam expander system is used for generating a beam having such a large diameter, thus making the focused beam diameter variable by changing the optical magnification of the beam expander system.
Since the first method uses an aperture stop to shade a portion of the incident beam so that the focused beam diameter can be expanded, incident beam utilization is reduced. The focused beam diameter is expanded in order to reduce the scanning density and so that the recording density is made rough. Although the quantity of light required for expanding the focused beam diameter is larger than that required when high-density recording is carried out with a small focused beam diameter, the quantity of light to be shaded by the aperture stop is actually increased and the quantity of light to be used as the focused beam is reduced.
On the other hand, the second method requires use of plural optical systems with plural lenses or a zoom system as the expander system. For this reason, the optical systems and the associated movable adjusting mechanism are made complicated, and besides, they are expensive.